ABSTRACT
It was first established by grafting experiments that neural induction occurs in Xenopus laevis and that it is the mesoderm in the dorsal lip of the blastopore which normally exercises this function. The subsequent histological work provided the following information:
At stage 
mesodermal invagination was already well under way, in advance of the formation of the archenteric cavity. This confirms the earlier observations of Nieuwkoop & Florschutz (1950).
The first evidence of neural induction, thickening of the mid-dorsal ectoderm combined with the development of an inner tier of columnar cells, occurred at stage 
.
By stage .12 there was generalized thickening of the dorsal ectoderm and between stage 
and 13 the brain and spinal cord regions of the neural plate became distinguishable.
The dorsal mesoderm segregated into notochord rudiment and two lateral masses at stage 13 and the latter further subdivided into paraxial mesoderm and lateral plates by stage 14.
The margins of the neural plate were clearly distinguished from presumptive epidermis by stage 15 and the median neural groove was also well marked.
In the next two stages the folding of the neural plate in the line of this groove proceeded rapidly. The dorsoventral enlargement of the somites and the relative shrinkage of the noto-chord were considered to contribute to the mechanism of neurulation.
Regionalization of the brain into prosencephalon, mesencephalon and rhombencephalon was in progress at stages 18 and 19.
These results indicate that induction consists of an initial activation of dorsal ectoderm (generalized thickening) followed by gradual transformation of the neural plate to form the different parts of the central nervous system (regionalization).
Intercellular metachromatic material was noted in various parts of the embryo. This was most plentiful between stage 
and stage 13 and thereafter gradually decreased. It was the only feature which persisted long enough to represent a possible inductive agent.
At all stages the archenteron was lined with a continuous layer of endoderm. This indicates that the mode of formation of the gastro-intestinal tube in Xenopus is different to that in urodeles. It further implies that the mesoderm is not present on the blastular surface prior to gastrulation but lies in deeper layers.
